94 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 148 



surface, relatively narrow, about 0.3 times the glabellar length and 

 equaling the distance to posterior cranidial margin. Posterior area 

 about as wide (tr.) as occipital ring. Free cheek with slightly convex 

 border defined by shallow furrow; genal spine oval in cross section, 

 fairly long, tapering to sharp point. 



Pygidium associated with the cranidia transversely subelliptical, 

 twice as wide as long. Axis prominent, showing 2 rings plus a termi- 

 nal section, almost reaching margin. Pleural lobes with 2 more or less 

 distinct pairs of broad furrows and very weak interpleural grooves. 

 Anterior outline regularly curved, anterior angles rounded. Border 

 furrow and border undefined. 



Surface of test smooth. Length of largest cranidium (exclusive of 

 spine) 7 mm. Length of pygidium 2 mm., width 4 mm. 



Discussion. — The assignment of the species to Aphelaspis was justi- 

 fied in the discussion of the genus. Aphelaspis arses may be distin- 

 guished from most species of the genus, apart from the occipital spine, 

 by the better than average definition of the occipital ring and palpebral 

 lobe. The closest relatives are two species described herein, A. arsoides 

 and A. inermis; the differences are mentioned in their discussion. 



Occurrence. — The type locality is U.S.N.M. 173, near Maryville. 

 The exposures in that area were very poor and may now be concealed 

 in built-up areas. The author's material was collected from localities 

 cnr/4 and cns/4. Lost Creek, cns/20, Washburn, and cnsVlS, Three 

 Springs. 



Types.— Holotype: U.S.N.M. 61617. Plesiotypes: U.S.N.M. 

 144676, 144733. 



APHELASPIS ARSOIDES Rasetti, new species 



Plate 11, figures 15-21 ; plate 12, figure 22 



Available material. — Large numbers of cranidia and free cheeks 

 from several localities, and a few pygidia. 



Description. — Glabella rather flat, defined by shallow axial furrow. 

 Occipital furrow shallow ; occipital ring extended into long, slender, 

 almost horizontal spine, not bearing a node. Frontal area from 0.5 to 

 0.7 times the glabellar length. Preglabellar field with very slight longi- 

 tudinal convexity ; border of average width, defined by furrow and 

 change in slope, more or less convex. Anterior sections of facial suture 

 straight to border furrow, diverging at somewhat greater angle than 

 in most species, including A. arses. Palpebral area horizontal to 

 slightly downsloping, about 0.3 times the glabellar width. Ocular 

 ridges distinct, slanted backward. Palpebral lobe almost undefined by 

 furrow on upper surface, about 0.3 times the glabellar length, and 

 equaling the distance to posterior margin. Posterior area somewhat 



